1. Field of the Invention
The present invention generally relates to antenna structures and, more particularly, is concerned with a slot antenna formed in a side wall of a metal housing and is designed for attaining minimum slot length with the use of a secondary slot and attaining resonance at two frequencies with the use of diodes connected across the slot at a pre-determined position.
2. Description of the Prior Art
Cavity-backed slot antennas are well known in the prior art. This type of antenna relies on the use of a slot in a metallic plane to radiate and capture electromagnetic waves. The classic description of a slot antenna is a narrow slot, equal in length to a half wavelength, cut into an infinite plane. In practice, the plane is not infinite, but experience shows that if the metal sheet boundaries are at least a quarter wavelength from the slot, the slot will behave nearly as if the metal plane dimensions were very large, and the slot length will be close to a half wavelength in free space. However, if the width and depth (or the radius, in the case of a cylindrical box) are much smaller than a quarter wavelength, it is found that it is not possible to obtain a resonant slot with a box length which is close to the free-space half wavelength. The box must be much longer. Traditionally, cavity-backed slot antennas are composed of a metal surface backed by an energized resonant cavity and having a slot through which energy is radiated directionally. Representatives of the prior art are the cavity-backed slot antennas disclosed in U.S. Pat. Nos. Mussler (U.S. Pat. No. 4,733,245), Mori et al. (U.S. Pat. No. 4,935,745), Lee (U.S. Pat. No. 4,975,711), Woloszczuk (U.S. Pat. No. 4,983,986), Stang (U.S. Pat. No. 3,725,941), Koyama et al (U.S. Pat. No. 5,757,326), Monser (U.S. Pat. No. 4,132,995), Luedtke et al (U.S. Pat. No. 4,229,744) and Boubouleix (U.S. Pat. No. 4,491,843).
Various approaches have been proposed in certain of the above referred to patents to limit the physical size of the cavity-backed slot antenna and fine tune to the right frequencies. For instance, in Mussler (U.S. Pat. No. 4,733,245), size reduction of the slot antenna cavity is achieved primarily through use of a high dielectric constant layer placed at the radiating portion of the antenna. Also, Mussler suggests that small value capacitors be employed to permit "fine tuning" to the frequency wanted. In Lee (U.S. Pat. No. 4,975,711), two slot antennas are used, one for vertical-polarized-wave and another for slant vertical-polarized or horizontal-polarized waves. In Mori, et al. (U.S. Pat. No. 4,935,745), the antenna is formed by three plates arranged to have a generally U-shaped cross section. Again, a capacitor is used to adjust the operation frequency of the antenna.
The cavity-backed slot antennas of the cited prior art which are identified above as ones concerned with space reduction and fine tuning to the right frequencies would appear to operate reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed. However, they do provide opportunities for further improvements to be made in terms of reduction of the complexity and constraints they introduced into their antenna designs to achieve the objective of reduced size. Consequently, a need still exists for improvements in cavity-backed slot antenna design that will make size reduction and resonance at two frequencies possible without introducing other factors that will diminish antenna performance and increase complexity and cost.